Barack Obama has clinched the Democratic nomination for president, becoming the first African-American in the US history to lead a major-party ticket.

From a dark horse in the Democrat race to the first black US presidential candidate from a major party, Obama has had a meteoric rise from political obscurity to be at a sniffing distance of the White House.

The 46-year-old Harvard-educated first-time Senator from Illinois had a prolonged bitter battle with powerful Democrat rival Hillary Clinton for winning the nomination -- a roller-coaster run that was dominated by frequent controversies, mostly related to his race and religion.

Son of a father who travelled from a small Kenyan village to pursue University education in Hawaii and went on to marry a white woman from Kansas, Obama started his political career as a low-paid community organiser.